A conventional orthogonal excitation gas laser oscillator has an oscillator housing of a hermetically sealed structure in which laser mediating gas such as CO2 gas is sealed, and is provided in the oscillator housing with discharge electrodes for discharging and exciting the laser mediating gas, heat exchangers for cooling the laser mediating gas, and a fan for circulating the laser mediating gas. Moreover, a mirror optical system that configures a gas laser oscillator resonator is provided in the two sides of the oscillator housing. In the continuous oscillation of gas laser such as CO2 gas laser, discharge excitation, in which excitation occurs by electron collision at the time of discharging, is generally used to excite (pump) gas molecules that emit laser light to an energy level required for induction and discharge. In such a situation, to ensure stable discharging, the inside of the oscillator housing needs to be kept under vacuum of 30 to 60 Torr, which means that the oscillator housing of the gas laser oscillator needs to have a tight sealing property that can keep the vacuum state.
Furthermore, because regular maintenance needs to be performed for the discharge electrodes, the heat exchangers, the fan, the mirror optical system, and the like that are arranged inside the oscillator housing, it is preferable that these are easily accessible for a maintenance person from the outside of the oscillator housing. For this reason, in general, a large opening is provided in the oscillator housing, and a removable cover unit is arranged to cover this opening.
Because of the above, the cover unit of the gas laser oscillator preferred to be designed to sustain a load applied when the inside of the oscillator housing is brought to a vacuum state, to have an air tight property to maintain the vacuum state, and also to keep an opening whose area is large enough to facilitate the maintenance for the inside components of the oscillator housing. In addition, the laser oscillator, which is used mainly for laser processing, has been developed to produce higher power so that the processing capacity can be improved. For this reason, long discharge electrodes, large heat exchangers, and large gas ducts are introduced, and as a result, the housing becomes longer especially in the optical axis direction and its height and width are also increased. Moreover, because a larger atmospheric pressure is exerted thereon due to the surface area that is increased in accordance with the upsizing, greater strength and rigidity are required for the housing and the cover unit against it.
Hence, according to the conventional technology, in a laser oscillator that includes a rectangular-solid housing in which structural devices of a gas laser oscillator such as discharge electrodes and mirrors, ribs and opening supporting components arranged in the housing to prevent the housing from being deformed, and a cover unit arranged to cover the opening of the housing, a structure in which a curved surface expanding toward either inside or outside of the housing is formed in the cover unit has been suggested (see, for example, Patent Literature 1).